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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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where intense glacial erosion has removed the ledges. The second condition is where these cornice glaciers seem to be remnants of the thinner, shrinking valley glaciers of a region, and are not nourished where they rest but are slowly wasting away or remain nearly in balance between snowfall and wastage. Both types contain true glacial ice, whose blue or green may be seen in crevasses. They often move forward to cliff edges, where fragments tumble to valleys below. They grade into the snowbank without ice, on the one hand, and into the true valley glacier on the other.
The Valley Glacier. The Alaskan valley glacier is in no fundamental respect different from its counterpart in the Alps, Caucasus, Himalayas, and other mountain regions, and may, therefore, be considered normal. There are, of course, many differences in reservoir conditions, in number and nature of tributaries, in grade, in rate of flow, in extent of crevassing, in size, and in other less important respects. A few in the Yakutat Bay region have their sources in normal cirque reservoirs among lofty mountains; more flow from the steeply-inclined snowfield areas on precipitous mountain tops and slopes; some head on the broad flat divides of through glacier systems. There are so many glaciers that do not head in cirques, the snow supply is so heavy, and avalanches from valley heads and walls are so frequent that the bergschrund is not always well defined near Yakutat Bay.
The glaciers are usually enclosed between steeply-rising ice-eroded mountain slopes, from which both snow and rock are frequently avalanched upon their surfaces. The snow line lies from 2000 to 3000 feet above sea level, and in all the larger glaciers a part of the actively-moving ice stream lies above snow line, so that the neV6 region is likely to be concealed beneath the lower end of the snowfield. The glacier is supplied from upper reservoirs, from snowfall upon the glacier ice below these, from the avalanching of snow from steeply-rising enclosing mountain walls, both in and below the reservoir zone, and from tributaries. In this region of heavy snowfall the glaciers are so well supplied that even small tongues push their termini practically to sea level, while larger ones either enter the sea or spread out to the mountain base.
The valley glacier has the normal morainic load common to such glaciers. There are medial moraines here and there, and lateral moraines are almost universally present, while the lower ice layers are heavily debris-charged. In some of the glaciers conditions of rock supply and ablation are so favorable that a sheet of debris coats the lower glacier from side to side, forming the broad fields of ablation moraine that are more fully described and discussed in a later section. In the dissipator, lateral moraines become etched into relief and a marginal valley commonly develops on either margin of the glacier, in which streams flow and deposits of complex nature are being accumulated. These marginal conditions are absent in only a few cases, the three most pronounced being
(1)  where recent spasmodic advance has closed up marginal valleys previously existing,
(2)  where the glacier terminates in a cascade on the steep face of the valley wall, and
(3) where the combination of ablation and iceberg discharge in an actively-moving valley-enclosed glacier, cuts back into the dissipator too rapidly for the development of marginal valleys, as is illustrated in Nunatak Glacier.
The deposits which would be left in these mountain valleys if the valley glaciers should melt out of them, would, about the termini, be closely like those of other valley glacier regions. There would be a fairly thin sheet of ground moraine, more or less well-defined medial and lateral moraine bands, and marginal valley deposits of varying character, extent,.